Weather, Water & Energy

As noted earlier in this module, water is the only common substance on Earth that occurs in all three states of matter: liquid, solid (ice), and gas (water vapor). But how does water change from these different states?

 

Phase Changes

Phase changes occur when water changes from one state of matter to another. Each of these phase changes has a name. The four most common phase changes are:

  • Melting: solid to liquid
  • Freezing: liquid to solid
  • Condensation: water vapor to liquid (cloud formation)
  • Evaporation: liquid to water vapor

Less common are the phase changes between solid (ice) and gaseous (water vapor):

  • Deposition: water vapor to solid – this happens when water vapor attaches directly to an ice crystal. This is the process that leads to the formation of frost, and also occurs in your freezer to create ice build-up.
  • Sublimation: solid to water vapor – the classic example of this is the evaporation of dry ice. Dry ice is frozen carbon dioxide; when exposed to air, dry ice immediately evaporates to gas. A more natural example that actually involves water (not CO2) would be shrinking snowpacks in dry, windy environments, like an exposed, high-altitude mountain ridge.

 

Phase Changes and Energy

When a phase change occurs, heat energy is absorbed or released. This energy is what powers the circulation of the atmosphere and drives daily weather patterns and the hydrologic cycle!

Each phase change involves absorbing or releasing energy. For instance, melting (solid to liquid) involves absorbing energy, while freezing (liquid to solid) involves releasing energy.
Each phase change involves absorbing or releasing energy. For instance, melting (solid to liquid) involves absorbing energy, while freezing (liquid to solid) involves releasing energy.

 

Take for example a lake at 20 degrees Celsius under typical conditions. One gram of water in the lake would need to absorb 585 calories (heat increase) to evaporate. Conversely, one gram of water vapor in the atmosphere would need to release 585 calories (heat decrease) for condensation to occur, allowing the water to precipitate back into the lake.

 

Liquid water in a lake evaporates when it absorbs energy due to increasing heat. Water vapor undergoes condensation - changing from a gaseous to liquid state - when it releases heat energy due to decreasing temperatures at higher altitudes.
Liquid water in a lake evaporates when it absorbs energy due to increasing heat. Water vapor undergoes condensation - changing from a gaseous to liquid state - when it releases heat energy due to decreasing temperatures at higher altitudes.

 

Another way to think about the role of energy in the phase change between evaporation and condensation the example of boiling a pot of water on a stove. You are applying heat energy to that water! As water boils, it evaporates - think about steam floating up from that boiling pot. If you leave the pot boiling long enough, all the water will boil away and you might destroy that pot!

Meanwhile, if that steam (a.k.a. water vapor) touches a cooler surface - say, a window next to your stove - the lower temperature of that surface will cause the steam to undergo condensation, and water droplets will form on that window. This is also why mirrors fog up in the bathroom when you take a hot shower - the steam from that water is condensing on the cooler mirror surface!

Likewise, putting an ice cube tray full of liquid water into a freezer will remove heat energy, resulting in freezing - the phase change from liquid water to solid ice. Conversely, taking ice cubes out of the freezer and leaving them in the warmer kitchen will cause melting - the phase change from solid (ice) to liquid (water).